1. Field of the Invention
The present invention relates to a light exposure system used for fabricating a semiconductor device. More particularly, the present invention relates to an apparatus for measuring the focus of the light exposure system.
2. Description of the Related Art
Measurement of the focus of a light exposure system is very important, because the amount of the defocusing critically affects the shape and size of the pattern formed on a photosensitive plate. As the operational method of the exposure system has been recently changed from the so-called xe2x80x9cstep and repeatxe2x80x9d type to the xe2x80x9cstep and scanxe2x80x9d type, the measurement and control of the focus require more highly advanced technology. The step and repeat type method exposes the photosensitive plate in static state after adjusting the focus where as, the step and scan type method must continue measuring and adjusting the focus because the photosensitive plate is moving while exposed.
The apparatus for measuring the focus of a light exposure system is basically an instrument for measuring a distance. The methods of measuring the focus in the exposure system are generally classified into two groups. One group is based on constructing an optical interference using a light source such as laser, and the other group is based on measuring the capacitance between the photosensitive plate and the electrode. The former method may correctly read only the light source exposed portion to measure the focus, and therefore, by selecting a number of measuring points, the surface state of the photosensitive plate may be correctly detected together with the tilt or leveling of an exposed region. However, in this case, the coverage of the measuring light source is very small compared to the exposed region and so allows only a localized measurement. Thus, an erroneous result may occur depending on the optical properties of the photosensitive plate with respect to the surface thin film. Hence, if the point measure of the focus accidentally does not belong to the representative group, the resultant pattern may be defocused.
Meanwhile, the latter method measuring the capacitance easily covers a large area to measure the focus, and therefore it is possible to correctly measure the average focus of the whole regions exposed. The step and scan type performs the exposure especially through a narrow slit adversely affecting the measurement of the tilt of the exposed region. However, it is advantageous in that the measurement is determined by the distance between the electrode and the photosensitive plate and is not affected significantly by the surface state of the photosensitive plate. The conventional apparatus for measuring such capacitance includes a grounded conductive stage, photosensitive plate closely contacting the upper surface of the stage, and focus measuring part having an exposing slit and opaque electrodes mounted around the slit and arranged over the stage.
Referring to FIG. 1, the conventional focus measuring part 10 is divided into the transparent and opaque regions. The transparent region is provided with a rectangular slit 11 while the opaque region is provided with first, second, third and fourth opaque electrodes 12, 13, 14 and 15. The opaque electrodes have been conventionally used to measure respective capacitances, of which the effective values within a predetermined range are used to obtain the measurement average.
However, the conventional apparatus for measuring the focus suffers the inherent structural problem that the opaque electrodes used for measuring the focus are separated from the transparent region, which tends to degrade the accuracy of measurement. Namely, the exposed region is displaced from the transparent region with reference to the photosensitive plate. Thus, if the surface of the photosensitive plate has a slight slope the measurement error will be insignificant, if however, the photosensitive plate is abruptly sloped the error will be significant. Such abrupt slope often occurs in the chip boundary of the photosensitive plate, so that the chip arranged in the edges of the photosensitive plate may suffer pattern imperfection.
It is a feature of an embodiment of the present invention to provide an apparatus for measuring the focus of a light exposure system to minimize the measurement error.
According to an aspect of the present invention, an apparatus for measuring the focus of a light exposure system used for selectively exposing a photosensitive plate to light rays in a process of fabricating a semiconductor device, comprises a focus measuring part having opaque region, transparent region, and a transparent electrode arranged in the transparent region, a stage supporting the photosensitive plate; and a capacitance detector for measuring the capacitance between the transparent electrode and the stage.
These and other features of the present invention will be readily apparent to those of ordinary skill in the art upon review of the detailed description that follows. The present invention will now be described more specifically with reference to the drawings attached only by way of example.